var VSHADER_SOURCE = `
attribute vec4 a_Position;
attribute vec4 a_Color;
uniform mat4 u_MvpMatrix;
varying vec4 v_Color;
void main(){
    gl_Position=u_MvpMatrix*a_Position;
    v_Color=a_Color;
}`;
var FSHADER_SOURCE =
    `precision mediump float;
    varying vec4 v_Color;
    void main(){
        gl_FragColor=v_Color;
    }`;

function main() {
    var canvas = document.getElementById('webgl');
    var gl = getWebGLContext(canvas);
    if (!gl) {
        console.log('Failed to get the rendering context for WebGL');
        return;
    }

    if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
        console.log('Failed to initialize shaders');
        return;
    }

    var n = initVerttexBuffers(gl);
    if (n < 0) {
        console.log('Failed to set the position of the vertices');
        return;
    }
    var u_MvpMatrix = gl.getUniformLocation(gl.program, 'u_MvpMatrix');
    if (u_MvpMatrix < 0) {
        console.log('Failed to get the location of the u_MvpMatrix');
        return;
    }

    var mvpMatrix = new Matrix4();
    mvpMatrix.setPerspective(30, 1, 1, 100);
    mvpMatrix.lookAt(3, 3, 7, 0, 0, 0, 0, 1, 0);

    gl.uniformMatrix4fv(u_MvpMatrix, false, mvpMatrix.elements);

    gl.enable(gl.DEPTH_TEST);
    gl.clearColor(0.0, 0.0, 0.0, 1.0);
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
    gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);

}
var g_points = [];

function initVerttexBuffers(gl) {
    // Create a cube
    //    v6----- v5
    //   /|      /|
    //  v1------v0|
    //  | |     | |
    //  | |v7---|-|v4
    //  |/      |/
    //  v2------v3
    // var verticesColors = new Float32Array([
    //     // Vertex coordinates and color
    //     1.0, 1.0, 1.0, 1.0, 1.0, 1.0, // v0 White
    //     -1.0, 1.0, 1.0, 1.0, 0.0, 1.0, // v1 Magenta
    //     -1.0, -1.0, 1.0, 1.0, 0.0, 0.0, // v2 Red
    //     1.0, -1.0, 1.0, 1.0, 1.0, 0.0, // v3 Yellow
    //     1.0, -1.0, -1.0, 0.0, 1.0, 0.0, // v4 Green
    //     1.0, 1.0, -1.0, 0.0, 1.0, 1.0, // v5 Cyan
    //     -1.0, 1.0, -1.0, 0.0, 0.0, 1.0, // v6 Blue
    //     -1.0, -1.0, -1.0, 0.0, 0.0, 0.0 // v7 Black
    // ]);

    var verticesColors = new Float32Array([
        // Vertex coordinates and color
        1.0, 1.0, 1.0, 1.0, 1.0, 1.0, -1.0, 1.0, 1.0, 1.0, 1.0, 1.0, -1.0, -1.0, 1.0, 1.0, 1.0, 1.0,
        1.0, -1.0, 1.0, 1.0, 1.0, 1.0,
        1.0, -1.0, -1.0, 1.0, 1.0, 1.0,
        1.0, 1.0, -1.0, 1.0, 1.0, 1.0, -1.0, 1.0, -1.0, 1.0, 1.0, 1.0, -1.0, -1.0, -1.0, 1.0, 1.0, 1.0
    ]);

    // Indices of the vertices
    var indices = new Uint8Array([
        0, 1, 2, 0, 2, 3, // front
        0, 3, 4, 0, 4, 5, // right
        0, 5, 6, 0, 6, 1, // up
        1, 6, 7, 1, 7, 2, // left
        7, 4, 3, 7, 3, 2, // down
        4, 7, 6, 4, 6, 5 // back
    ]);

    var FSIZE = verticesColors.BYTES_PER_ELEMENT;
    var vertexColorBuffer = gl.createBuffer();
    if (!vertexColorBuffer) {
        console.log('Failed to create the buffer object');
        return -1;
    }
    var indexBuffer = gl.createBuffer();
    if (!indexBuffer) {
        console.log('Failed to create the buffer object');
        return -1;
    }
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexColorBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, verticesColors, gl.STATIC_DRAW);

    var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
    if (a_Position < 0) {
        console.log('Failed to get the storge location of a_Position');
        return;
    }
    var a_Color = gl.getAttribLocation(gl.program, 'a_Color');
    if (a_Color < 0) {
        console.log('Failed to get the storge location of a_Color');
        return;
    }
    gl.vertexAttribPointer(a_Position, 3, gl.FLOAT, false, FSIZE * 6, 0);
    gl.enableVertexAttribArray(a_Position);
    gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, FSIZE * 6, FSIZE * 3);
    gl.enableVertexAttribArray(a_Color);

    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);
    gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, indices, gl.STATIC_DRAW);


    return indices.length;
}